This invention concerns polymers and paints. More particularly it concerns polyesterepoxy polymers particularly suited for use in chip resistant coating compositions for use on automobiles.
Coating systems for automobiles normally comprise a multiplicity of coatings applied to a steel substrate. Typically, the steel is treated with a phosphate, then a cathodic electrocoat primer is applied. A primer-surfacer is used next to smooth the surface and provide a thick enough coating to permit sanding to a smooth, flat finish. Then a top-coat system is applied, sometimes as a single colored coat, often as a base coat with solid color or metallic pigments followed by a clear coat.
In automobiles, a critical area for such coating systems is the lower part of the car including the rocker panels. Stones and debris from the road can cause chipping of the paint, particularly in those areas.
One solution to the chipping problem is to use a relatively thick, soft, mastick-like composition as the primer surfacer. However, the edge between such a thick coating and the thinner, smooth finish on the rest of the car is apt to create an appearance problem for the designers. While the edge can be masked with trim or molding, it would be desirable to have a thin chip-resistant primer surfacer with less of such interface problems.
The task of developing a thin, chip-resistant paint is difficult. Many paint resin systems are too brittle for such purposes. And the normal pigmentation of such paints often contributes to brittleness.